1. Field of the Invention
The present invention relates to scanning probe microscopes represented by a scanning tunneling microscope (STM), atomic force microscope (AFM), etc., and more specifically, to a scanning probe microscope, such as a scanning tunneling spectroscopic microscope or scanning tunneling potentiometric microscopes for measuring the electrical properties of the surface of a sample.
2. Description of the Related Art
One of scanning probe microscopes (SPMs) is proposed in U.S. Pat. No. Re. 33,387 entitled "Atomic Force Microscope and Method for Imaging Surfaces with Atomic Resolution." The SPMs have high resolutions on the atomic size level with respect to the vertical and horizontal directions despite their simple construction, and are represented by a scanning tunneling microscope (STM), atomic force microscope (AFM), etc.
Since the AFM was devised by G. Binnig et al., the inventors of the STM, (see "Physical Review Letters" vol. 56, p 930, 1986), it has been expected as novel surface configuration observing means for insulating materials and further investigated. The AFM has a probe, which is supported by means of a flexible cantilever. As the probe is brought close to the surface of a sample, a Van der Waals force first acts between the distal end of the probe and the sample surface. When the distance between the probe and the sample surface approaches the atomic distance, a repulsion force based on the Pauli exclusion principle then acts. The cantilever is displaced depending on the magnitude of an atomic force the distal end of the probe receives. Thus, an image of indentations of the sample surface can be obtained by scanning the sample surface with the probe and detecting the displacement of the cantilever between various points.
The STM moves the probe for scanning while keeping a tunneling current flowing between the probe and the sample constant by applying a bias voltage between the two, and obtains a servo signal for controlling the distance between the probe and the sample. The servo signal contains STM information, that is, reflects indentation information for the sample surface. Thus, an STM image with resolutions on the atomic size level can be obtained by plotting the STM information corresponding to the coordinates of measuring points.
The tunneling current reflects the distance between the probe and the sample, local state of electrons on the sample, and local potential of the sample. Accordingly, a normal STM image contains indentation information indicative of the microscopic roughness of the sample surface and information for the local potential distribution on the sample surface.
Recently, there have been developed scanning tunneling spectroscopy (STS) and scanning tunneling potentiometry (STP). In the STS, the indentation information and electronic property information for the sample surface are separated from the tunneling current, whereby information for the state of electrons on the surface are extracted. In the STP, the information for the potential distribution on the sample surface is extracted from the tunneling current.
Current imaging tunneling spectroscopy (CITS) is a representative of digital STS. In the CITS, the local density distribution of electrons on the sample surface is measured in accordance with the dependence of the tunneling current on the bias voltage. The CITS is based on the fact that the differential conductance is proportional to the local electron density in the case where the tunneling gap (distance between sample and probe) and barrier height are fixed without regard to the location. Thus, in the CITS, local current and voltage values are obtained as the probe is moved for scanning, they are stored in advance for each measuring point, and the differential conductance is obtained later by numerical computation.
The STS and STP measurements are carried out by using a probe for STM measurement on the assumption that the distance between the probe and the sample is fixed. The probe is relatively moved for XY-scanning over the surface of the sample in parallel relation throughout a measuring region of the surface. In the scanning operation, the probe is temporarily stopped at each of a large number of measuring points that are previously suitably arranged on a scanning line, and the electrical properties of the sample surface at each measuring point are measured by means of the probe. The measurement of the electrical properties at each measuring point is made in a manner such that the distance between the probe and the sample is fixed at a predetermined value. More specifically, the distance between the probe and the sample is adjusted and fixed to a length equivalent to a predetermined value of the tunneling current flowing through the probe. Then, the electrical properties of the sample surface at each measuring point is determined by examining the value of the tunneling current flowing through the probe while changing the voltage applied between the probe and the sample.
While a piezoelectric scanner is used to control the distance between the probe and the sample, it tends to undergo creeping or drift.
For these reasons, the distance between the probe and the sample cannot be kept fixed during the STS or STP measurement.